This invention relates to a system and method for removing volatile components from an aqueous medium. More particularly, the invention is directed to removing dissolved impurities from water by steam-stripping the impurities under deep vacuum conditions.
A major problem today is contamination of groundwater by hydrocarbons, such as gasoline, benzene, toluene, or xylene, that leak out of underground storage tanks. Groundwater can also be contaminated by chlorinated solvents, such as trichloroethylene, trichloroethane, perchloroethylene, and methylene chloride, which are used in degreasing operations and then dumped into open pits in the ground.
One way to remove organic impurities from groundwater is by a technique referred to as steam-stripping. In a typical steam-stripping process, the contaminated water is first passed through a heat exchanger to bring its temperature up to about 180.degree.-200.degree. F. and then it is directed into the top of a stripper column. Steam, as the vaporizing gas, enters the column near the bottom and flows upwardly at about 212.degree.-240.degree. F. As the downwardly moving water and the upwardly moving steam make contact inside the tower, the steam and water are at about the same temperature until the steam begins to contact the cooler incoming feed water near the top of the column.
At this point the steam condenses on the cooler feed water to heat the water to its boiling point. Since much of the steam is consumed in heating the cooler feed water, only a very small amount of steam is available for vaporizing the hydrocarbon impurities and carrying the impurities out of the column in the vapor phase. As the vapor phase leaves the column, it is passed through a heat exchanger to condense the vapors to a mixture of water and the organic impurities (oil). The oil is separated from the water in a decanter vessel and the oil phase is discharged to an incinerator, or carried to another point for further processing. The water phase, which is still saturated with dissolved impurities, is recycled back to the column and stripped again to remove the impurities.
The steam-stripping procedure described above has a major drawback. As pointed out in the description, only a small amount of steam is available, as a vaporizing gas, to carry the impurities out of the stripper column. Therefore, the amount of steam required to operate under such conditions is relatively high and the process is costly to operate. The present invention overcomes this problem by operating the stripper system at a reduced pressure, that is, under a vacuum condition, so that the feed water can enter the column at its boiling point.